1. Field of the Invention
The present invention relates to a digital image data correction apparatus and a digital image data correction method for correcting optical distortion of digital image data, and a digital image pickup apparatus having the digital image data correction apparatus.
2. Description of the Related Art
An image pickup apparatus includes an apparatus for picking up an object to be photographed through an optical lens of a silver halide camera, a digital camera or the like, and obtaining an image showing the object. In this image pickup apparatus, distortion occurs around the obtained image by refraction of the optical lens. The distortion is generally called optical distortion. The lens used in the image pickup apparatus is constituted so as to cancel the optical distortion. In the case of a zoom lens, however, it is difficult to correct both a telephoto end and a wide end with the same lens configuration. Large optical distortion, therefore, easily occurs. Even a single-focal lens requires an expensive lens material or a thicker lens configuration for correction. It is, therefore, difficult to provide a thin and economical lens configuration. As a result, the optical distortion still remains.
When an image obtained is recorded on a film with a silver halide camera, the recorded image cannot be corrected. The optical distortion is, therefore, determined by a lens performance. On the contrary, when an image is obtained by digital data and is recorded onto recording media like the digital camera, the image can be corrected by an operating process even after recording. In the digital camera field, therefore, conventionally, techniques for the correction of optical distortion have been proposed.
One technique for correcting such optical distortion includes a method of representing a correcting amount by an approximation formula so as to perform correction as described in Japanese Patent Application Laid-Open (JP-A) Nos. 11-250238, 11-250239, 11-250240 and 11-252431. It is generally known that an amount of data conversion due to the optical distortion can be approximately expressed by a polynomial depending on a distance from an optical center. In this method, therefore, an inverse number of the polynomial is used as a correcting expression. Concretely, a coordinate on an uncorrected image corresponding to each pixel on a corrected image is calculated based on the polynomial by a CPU. The each pixel is interpolated with pixel data around the calculated coordinate, so that correction is performed.
In the prior technique, however, when the interpolation is carried out by bi-linear interpolation, for example, an operation using four-pixel data around the calculated coordinate is necessary. The operation for the correction becomes, therefore, complicated. Accordingly, the CPU needs to do a large amount of calculation only for the processing of one pixel. There, therefore, arises such a problem that the processing of an entire image takes much processing time. Particularly when the number of pixels of the digital image data is increased, the time required for the correction also increases. In order to achieve further high resolution with a digital image pickup apparatus, therefore, reducing of the correcting process time is demanded.